A capacity-variable compressor (variable displacement compressor) used in a refrigeration cycle device for a vehicle is driven by an engine of the vehicle as a source of drive, and is provided with a capacity-varying unit for varying the refrigerant discharge capacity. In the refrigeration cycle device used in an air condition control unit, the refrigerant discharge capacity of the capacity-variable compressor is controlled by operating the capacity-varying unit so that the suction pressure of the capacity-variable compressor becomes equal to a preset suction pressure.
In the known refrigeration cycle device for a vehicle equipped with the capacity-variable compressor of the suction pressure control type, an emergency control is performed such as acceleration cut control of the capacity-variable compressor in a transient state where the vehicle speed sharply changes and the rotational speed of the compressor sharply changes, in order to decrease the load exerted on the engine of the vehicle (e.g., JP-A-2001-47856).
In the above discharge capacity control, however, at the time of shifting into the normal control after the end of the acceleration cut control (emergency control), a control signal output to the capacity-varying unit is determined by selecting an optimum control map out of a plurality of control maps defining correlations between the control signals and a shift elapse time from the emergency control to the normal control. The control map is selected based on the compressor suction pressure at the end of the emergency control. Here, a suction pressure sensor for detecting the suction pressure is used to correctly detect the suction pressure of the compressor resulting, however, in an increase in the cost due to the constitution and arrangement in the compressor and due to an increase in the number of parts.
Therefore, when the refrigerant discharge capacity of the capacity-variable compressor is controlled, in general, the pressure in an evaporator is calculated from the temperature of the evaporator detected by an evaporator temperature sensor, and the pressure in the evaporator is estimated as the compressor suction pressure. In this case, a problem arouses in that the suction pressure of the compressor that is estimated from the evaporator pressure deviates from the real suction pressure of the compressor.